Window balance

A home owner from California, USA says:
My Window balance is 1/2" x 22" stamped with 2140 B.S.I...Not sure if I need 390 or 351 series. I'm leaning towards 390 but not sure what side I measure shoe.
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Casey from SWISCO responded:

Hello,

As long as the top fitting of your balance does not have wings, I agree that you could have either our S351-22 , with a 15-002 bottom attachment, or S390-22 , with a 15-001 bottom attachment. Take a look at the image below to determine which balance you'll need. You'll also need a 2140 spring strength option for your replacement balances. Thank you!

Comparison of the S351 and S390 balance shoes.
Comparison of the S351 and S390 balance shoes.
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